Deletes the node_modules folder to decrease slug size.
$ heroku buildpacks:set https://github.com/ztory/heroku-buildpack-node-cleanup.git
Or add it to the .buildpacks file if using heroku-buildpack-multi:
$ cat .buildpacks
...
https://github.com/ztory/heroku-buildpack-node-cleanup.git